Page 1 of 9 Abstract- Modern digital relays not only help to increase the speed, sensitivity and dependability for transformer protection, but also help to simplify the differential (87T) protection in circuit design and setting calculations. However, mis-operations of digital 87T relays still happen from time to time. This paper presents a few relay mis-operation cases that occurred in real life. One of the common lessons learned is: even though digital relays are superior and appears simpler for the user, the thinking process on protection can never be waived. In order to ensure the quality of relay settings, the relay engineer needs to know not only the protection fundamentals, but also needs to gain some insight of how digital relay works. I. INTRODUCTION The theory and practice of transformer protection has been mature and comprehensive for many years. But in reality, transformer protection mis-operations still happen from time to time. The common reasons of mis-operations are: ⋅ Incorrect settings of 87T function ⋅ Incorrect settings of transformer overcurrent protection – the setting is either too sensitive or lack of coordination with adjacent lines or feeders ⋅ Inrush current during energization or voltage recovery ⋅ CT polarity error in design or construction ⋅ False operation of non-electric protection, such as sudden pressure relays, Buchholz relay, etc. ⋅ Relay failure This paper will focus on the first type of mis-operations. As the primary protection for large or mid-size transformers, the 87T function is considered to be reliable, sensitive, fast and selective. Before modern microprocessor relays, the 87T schemes built upon electromechanical (EM) relays were prone to human error mainly because of the auxiliary CTs used for current compensation. Modern digital relays have greatly simplified the 87T scheme with regards to secondary circuit design and settings, but setting-related mis-operations still happen from time to time. This paper presents a few cases that were caused by incorrect settings of digital 87T relays. In the first case, the mis-operation was caused by a setting mistake on differential current compensation. The second case is about a refurbish project in which the old 87T relay was replaced but the old circuit was maintained. In that project, the 87T settings of the digital relay were following the relay manual, but the mis-matching between the old circuit and the digital relay have created a mistake that was not easy to identify. The third mis-operation case may or may not be labeled as “user error”, since it was the “automatic” setting of the digital relay that caused the mis-operation under heavy through fault condition. The fourth case has power electronics involved and it reminds us of another possible source of error in differential current. The fifth mis-operation case appears to be caused by inrush current during energization. But it turns out to be the issue of CT ratio and 87T pickup setting. II. 87T FUNCTION OF A DIGITAL RELAY Like other digital relays, the 87T relay will perform filtering and phasor estimation as the first step of signal processing. After the current phasors are derived, they will be compensated before computing the two key quantities of the 87T function – the differential current (Id) and the restraint current (Ir). Most 87T relays would use the percentage characteristic to compare Id and Ir to determine if the fault is within the protection zone. Winding 1 Ph-A Current Winding 2 Ph-A Current Winding ‘n’ Ph-A Current Filtering and Phasor Calc. Filtering and Phasor Calc. Filtering and Phasor Calc. Magnitude, Phase angle and zero sequence compensation Magnitude, Phase angle and zero sequence compensation Magnitude, Phase angle and zero sequence compensation Maximum magnitude, or Sum of magnitude, etc. Vector Sum of Phasors Differential Current Iad Restraint Current Iar Id Ir Figure II.1. The 87T Implementation in a Digital Relay Fig.II.1 uses phase-A currents as an example to show the main signal flow of the 87T relay. The key step is the current compensation that includes magnitude compensation, phase angle compensation and the optional zero sequence removal. Different type of relays may have different implementations, but the purpose is the same - to achieve zero differential current during normal operation. In the EM relay era, the auxiliary CTs were used for the compensation, which may incur human error during design or construction. A digital 87T relay would use settings to simplify the compensation and is more secure by using the advanced 87T characteristic to override the spurious differential current caused by CT error, Mis-operation Cases on Transformer Differential Protection Yiyan Xue, Zachary Campbell, Sudhakar Chidurala, Charles Jones American Electric Power Company
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Page 1 of 9
Abstract- Modern digital relays not only help to increase the
speed, sensitivity and dependability for transformer protection,
but also help to simplify the differential (87T) protection in
circuit design and setting calculations. However, mis-operations
of digital 87T relays still happen from time to time. This paper
presents a few relay mis-operation cases that occurred in real life.
One of the common lessons learned is: even though digital relays
are superior and appears simpler for the user, the thinking
process on protection can never be waived. In order to ensure the
quality of relay settings, the relay engineer needs to know not
only the protection fundamentals, but also needs to gain some
insight of how digital relay works.
I. INTRODUCTION
The theory and practice of transformer protection has been
mature and comprehensive for many years. But in reality,
transformer protection mis-operations still happen from time
to time. The common reasons of mis-operations are:
⋅ Incorrect settings of 87T function
⋅ Incorrect settings of transformer overcurrent protection – the
setting is either too sensitive or lack of coordination with
adjacent lines or feeders
⋅ Inrush current during energization or voltage recovery
⋅ CT polarity error in design or construction
⋅ False operation of non-electric protection, such as sudden
pressure relays, Buchholz relay, etc.
⋅ Relay failure
This paper will focus on the first type of mis-operations. As
the primary protection for large or mid-size transformers, the
87T function is considered to be reliable, sensitive, fast and
selective. Before modern microprocessor relays, the 87T
schemes built upon electromechanical (EM) relays were prone
to human error mainly because of the auxiliary CTs used for
current compensation. Modern digital relays have greatly
simplified the 87T scheme with regards to secondary circuit
design and settings, but setting-related mis-operations still
happen from time to time. This paper presents a few cases that
were caused by incorrect settings of digital 87T relays.
In the first case, the mis-operation was caused by a setting
mistake on differential current compensation. The second case
is about a refurbish project in which the old 87T relay was
replaced but the old circuit was maintained. In that project, the
87T settings of the digital relay were following the relay
manual, but the mis-matching between the old circuit and the
digital relay have created a mistake that was not easy to
identify. The third mis-operation case may or may not be
labeled as “user error”, since it was the “automatic” setting of
the digital relay that caused the mis-operation under heavy
through fault condition. The fourth case has power electronics
involved and it reminds us of another possible source of error
in differential current. The fifth mis-operation case appears to
be caused by inrush current during energization. But it turns
out to be the issue of CT ratio and 87T pickup setting.
II. 87T FUNCTION OF A DIGITAL RELAY
Like other digital relays, the 87T relay will perform filtering
and phasor estimation as the first step of signal processing.
After the current phasors are derived, they will be
compensated before computing the two key quantities of the
87T function – the differential current (Id) and the restraint
current (Ir). Most 87T relays would use the percentage
characteristic to compare Id and Ir to determine if the fault is
within the protection zone.
Winding 1
Ph-A Current
Winding 2
Ph-A Current
Winding ‘n’
Ph-A Current
Filtering and
Phasor Calc.
Filtering and
Phasor Calc.
Filtering and
Phasor Calc.
Magnitude, Phase
angle and zero
sequence
compensation
Magnitude, Phase
angle and zero
sequence
compensation
Magnitude, Phase
angle and zero
sequence
compensation
Maximum magnitude, or
Sum of magnitude, etc.Vector Sum of Phasors
Differential
Current Iad
Restraint
Current Iar
Id
Ir
Figure II.1. The 87T Implementation in a Digital Relay
Fig.II.1 uses phase-A currents as an example to show the main
signal flow of the 87T relay. The key step is the current
compensation that includes magnitude compensation, phase
angle compensation and the optional zero sequence removal.
Different type of relays may have different implementations,
but the purpose is the same - to achieve zero differential
current during normal operation. In the EM relay era, the
auxiliary CTs were used for the compensation, which may
incur human error during design or construction. A digital 87T
relay would use settings to simplify the compensation and is
more secure by using the advanced 87T characteristic to
override the spurious differential current caused by CT error,
Mis-operation Cases on Transformer Differential Protection
Yiyan Xue, Zachary Campbell, Sudhakar Chidurala, Charles Jones
American Electric Power Company
Page 2 of 9
relay error, magnetizing current, tap changer, etc. Some digital
relay also incorporates an external fault detector that is based
on the detection of current change pattern. However, all the
advantages of digital relays are subject to a premise: the relay
settings have to be correct. In reality, setting errors related to
mis-operations happen from time to time. How to reduce
human error on settings is a challenge that both relay users and
manufacturers need to think about.
For an 87T relay, the setting error is usually associated with
current compensation. The following simple example explains
how current compensation works. In Fig. II.2, the transformer
138kV windings are connected in delta, 12kV windings are
wye-grounded. The CT’s at both sides are wye-grounded.
Figure II.2. The 87T Implementation in a Digital Relay
Most relays’ 87T function is on a per-phase basis. Using
phase-A as example, the relay will see the following currents
under nominal load condition,
AkV
MVAIH alno 615.2
400
5
1383
50min =⋅
⋅
=
AkV
MVAIL alno 01.4
3000
5
123
50min =⋅
⋅
=
Where ‘H’ and ‘L’ represents high and low voltage side
respectively. In order to get zero differential current under
normal condition, one method of magnitude compensation is
to use the nominal current as the conversion base, which is
also called TAP. In this case, TAPH = 2.615, TAPL = 4.01. So,
for any normal operation currents, the converted currents are
Nominal primary and secondary current for each winding•
Inorm_prim
Snom
Vnorm 3⋅
→ 261
523
448
A=:=
1
Inorm_sec
Inorm_prim
CTR
→ 2.615
5.23
4.481
A=:=
Tthe selection of CT ratio should make the secondary nominal current 1~5A for each winding.
3. Magnitude Compensation Factors
Imargin
CTprim
Inorm_prim
→ 1.912
0.956
1.116
=:=
The automatic reference winding is•
RefW_Auto match min Imargin( ) Imargin, ( ) 1+ 2( )=:=
The reference winding is•
RefW RefW_Auto0 ReferenceWinding 0=if
ReferenceWinding otherwise
2=:=
The magnitude compensation factors (M) are used to convert each winding current before the differential•and restraint current are calculated
MCTprim Vnorm⋅
CTprimRefW 1−
VnormRefW 1−
⋅
→ 2
1
0.667
=:=
If the setting of Reference Winding is "Automatic Selection", the winding cooresponding to the smallest MarginFactor will be selected by the relay as the reference winding. If you see a overly high M factor, please set thereference winding manually instead of using "Automatic Selection".
3. Phase Compensation Reference
i n 0←
n n 1+←
Connectionn "Y"=while
n
:=
i 2=
RefAngle AngWrtW1i 30−=:=
AngWrtRef RefAngle AngWrtW1−
30−
30−
0
=:=
4. Fault Analysis
2
Enter fault current phasors in primary value. The angle is in Degree.
IAFprim
175.85 ∠ 283.2−
83.6 ∠ 103.37−
80.52 ∠ 84.01−
A⋅:= IBFprim
112.84 ∠ 64.45−
102.95 ∠ 204.83−
592.83 ∠ 286.92−
A⋅:= ICFprim
112.9 ∠ 145.43−
119.15 ∠ 341.61−
236.57 ∠ 318.7−
A⋅:=
IAFIAFprim
CTR
→
:= IBFIBFprim
CTR
→
:= ICFICFprim
CTR
→
:=
Phase and zero sequence compensation•
IAp0gnd2 IAF⋅ IBF− ICF−
3:= IBp0gnd
2 IBF⋅ IAF− ICF−
3:= ICp0gnd
2 ICF⋅ IAF− IBF−
3:=
IBp30lagIBF IAF−
3:= ICp30lag
ICF IBF−
3:=
IAp30lagIAF ICF−
3:=
IBp30leadIBF ICF−
3:= ICp30lead
ICF IAF−
3:=
IAp30leadIAF IBF−
3:=
IAW n( ) s IAFn AngWrtRefn 0= Groundingn "Not Within Zone"=∧if
Y 1≡ D 3≡ pu 1≡ ∠∠∠∠ mag ang, ( ) mag cos ang deg⋅( ) i sin ang deg⋅( )⋅+( )⋅≡
The TxCTC setting represents one of the following matrixes. The CTC(12) produces no phase shift, but itremoves the zero-sequence components.
CTC121
3
2
1−
1−
1−
2
1−
1−
1−
2
⋅:= CTC11
3
1
0
1−
1−
1
0
0
1−
1
⋅:= CTC111
3
1
1−
0
0
1
1−
1−
0
1
⋅:=
D lags Y by 30 D leads Y by 30
1. Relay Settings
MVA_ 1500:= ICOM "Y":= Windings:S,T,U,W
CTRS 600:= CTCONS Y:= TSCTC CTC12:= VTERMS 765:=
CTRT 600:= CTCONT Y:= TTCTC CTC12:= VTERMT 765:=
CTRU 6928:= CTCONU D:= TUCTC CTC1:= VTERMU 26:=
CTRW 7000:= CTCONW Y:= TWCTC CTC1:= VTERMW 26:=
CTRX 1:= CTCONX Y:= TXCTC CTC1:= VTERMX 1:=
O87P 0.3:= U87P 16:=
SLP1 25%:= SLP2 60%:= DIOPR 1.2:= DIRTR 1.2:=
E87HB "N":= E87HR "Y":=
PCT2 20%:= PCT4 20%:= PCT5 35%:=
S87QP 1.0:= SLPQ1 100:= S87QD 100:=
2. Check the TAP
In order to compensate for the differential current due to CT ratios and transformer ratio, the relay will calculate thescaling factor, namely TAPn, for each winding current. The calculation method is,
⋅
1
TAPSMVA_ 1000⋅
3 VTERMS⋅ CTRS⋅CTCONS⋅ 1.89=:=
TAPTMVA_ 1000⋅
3 VTERMT⋅ CTRT⋅CTCONT⋅ 1.89=:=
TAPUMVA_ 1000⋅
3 VTERMU⋅ CTRU⋅CTCONU⋅ 8.33=:=
TAPWMVA_ 1000⋅
3 VTERMW⋅ CTRW⋅CTCONW⋅ 4.76=:=
TAPXMVA_ 1000⋅
3 VTERMX⋅ CTRX⋅CTCONX⋅ 866025.4=:=
2. 87T Calculation and Plots
Fault Current Phasors•
For each winding, enter the A, B, C phase current from top to bottom.